Carbon dioxide stunning is used for stunning pigs in many countries. There have been welfare concerns about CO2 because it is a pungent gas which is irritating to the respiratory tract (Gregory, 1994). One research lab reports that CO₂ is aversive to pigs (Raj et al, 1999). Another research team states that it is not aversive (Jong et al, 2000).

• Hoenderken (1982) reported that a motorific excitation phase occurs while the pig is still conscious.
• Forslid (1987) found that the excitation phase starts after the pig is unconscious in purebred Yorkshires.
• There is a large variation in a pig's reaction to CO2 (Dodman, 1977; Grandin, 1988a). The reactions range from none when the pigs first sniff the gas, to violent attempts to escape.
• Halothane-positive pigs have more excitation (Troegerand Waltersdorf, 1991).
• Carbon dioxide stunning may be a good method for certain genetic types of pigs and very stressful to others. Genetic differences probably explain differences in the reaction of pigs to CO₂.

Stress caused by forcing pigs to move through a single file race could be eliminated by stunning groups of pigs in CO2 gas. Barton Gade et al. (1993) has developed a low stress driving and lairage system for moving groups of five pigs onto an elevator which descends into CO2 gas. An entire system approach should be used for evaluating CO2 stunning. Some discomfort during the induction of anesthesia may be a small price to pay for great reductions in handling stress.

Effect of genetics on handling and CO2 stunning of pigs (with May 2000 updates)

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